1. Field of the Invention
The invention relates to amino-functional polyvinyl acetals, to processes for preparing them, and in particular, to their use in printing ink compositions.
2. Description of the Related Art
The preparation of polyvinyl acetals, which are obtained from the corresponding polyvinyl alcohols by polymer-analogous reaction with the corresponding aldehydes, has been known as early as 1924. Since then, a multiplicity of aldehydes have been used to prepare the corresponding polyvinyl acetals. Polyvinyl acetals are prepared in a three-stage process: polyvinyl acetate→polyvinyl alcohol→polyvinyl acetal, giving products which in addition to vinyl acetal groups, also include vinyl alcohol and vinyl acetate units. Commercial significance has been acquired in particular by polyvinyl formal, polyvinyl acetal and polyvinyl butyral (PVB).
The greatest area of application for polyvinyl acetals is in the production of safety glass in carmaking and in architecture, with plasticized polyvinyl butyral films being used as an interlayer in glass sheets. A further field of use for polyvinyl butyrals is in anti-corrosion coatings. Their good pigment-binding power is one of the reasons why polyvinyl butyrals are also used as binders in coating materials and especially in printing inks. In this application it is required that the organic solutions of the polyvinyl butyrals should have a very low solution viscosity, in order to allow the preparation of inks combining a high solids content with a very high binder fraction. Examples thereof are the modified polyvinyl butyrals with low solution viscosity from DE-A 19641064, which are obtained by acetalizing a copolymer having vinyl alcohol and 1-alkyl-vinyl alcohol units.
A disadvantage shared by all of the prior art polyvinyl acetals is their inadequate adhesion to specific substrates. Because of this it is mandatory in many cases to add adhesion promoters. EP-B 0346768 describes the coating of films or sheets with amino-functional silanes, such as N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, in order to improve their bonding with other polymers, especially with polyvinyl butyrals.
The prior art methods of improving adhesion between polyvinyl acetals and critical substrates by adding the known adhesion promoters have distinct disadvantages. For example, the adhesion promotion effect is often not maintained for a sufficient period, or the bond becomes weaker with time. Another problem is the addition of adhesion promoters themselves. It always entails an additional, time-consuming step during formulation. Moreover, the selection and addition of suitable adhesion promoters requires a great deal of know-how, since the addition of the adhesion promoter to a polyvinyl acetal—in an organic solvent, for example—may be accompanied by instances of incompatibility and inhomogeneity, possibly going as far as phase separation. Furthermore, side-reactions of the reactive silanes that are subsequently added as adhesion promoters cannot be ruled out (hydrolysis reactions, condensation reactions). A further disadvantage is the large amount of adhesion promoter that must usually be added in order to obtain a distinctly improved adhesion of polyvinyl acetals to critical substrates, and so this method, in view of the high price of silanes, can become very expensive.
EP-A 617166 A1 relates to the use of amino-functional polyvinyl alcohol for improving the properties of recycled paper. An alternative to the preparation is specified as being the reaction of polyvinyl alcohol with amino aldehyde dialkyl acetal. Besides the vinyl alcohol units, the only vinyl acetal units the product contains are amino acetal groups. A product of this kind is also known from WO 02/072361 A1, which describes amino-functional polyvinyl alcohols as binders for inkjet paper. EP 552702 A1 describes the use of vinylamine-vinyl alcohol copolymers, which where appropriate may have been acetalized, in papermaking. In the products specified therein the amine group is linked directly to the polymer chain. Products of the same kind, i.e. vinylamine-vinyl alcohol copolymers, which where appropriate additionally carry ester, amide and/or acetal groups, are described in EP 461399 A2.